The crosswise stretching of the fibrilled film is a difficult operation owing to the extreme lightness and delicacy of the material to be handled, because of transversal shrinkage phenomenon due to the crosswise opening and to the poor resistance of the fibrils to the longitudinal sliding stresses, all factors which have hindered the use of the conventional opening systems used in the field of textiles, such as diverging clamps or pins carrying chains, or other rigid diverging elements of kinds.
Various methods have been suggested in the art for this purpose. Said methods are however not completely suited for overcoming the above indicated drawbacks.
According to the method disclosed in French Pat. No. 1.331.095, said stretching is achieved by holding the film compressed between two stretchable and elastical films which are stretched crosswise with a simultaneous longitudinal shrinkage.
According to European Pat. No. 11.371 there is carried out an opening of tubularly shaped fibrilled films, by passing the films through an expansion form, and obtaining the compensation of the longitudinal shrinkage by using different feeding speeds and film winding-up speeds.
Lastly, in the European patent application No. 21.017, filed in the name of the Applicant, the opening of the fibrilled film is achieved by means of two sets of clamps flexibly connected to each other, which grip the film while it runs on two diverging guides, and which are dragged along by the film itself and are free to draw near each other during the opening of the film, thereby compensating the longitudinal shrinkage of the film itself.
Although this last method is particularly effective, it requests the use of particularly complex devices, to detriment of the economy of the production.